As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated sheet packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material, and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH), which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating. The web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The packaging machines comprise a forming unit, in which the tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which are then fed to a folding unit to form the finished, e.g. substantially parallelepiped-shaped packages.
More specifically, the pillow packs substantially comprise a parallelepiped-shaped main portion, and a top end portion and a bottom end portion, opposite to each other and projecting laterally on opposite sides of the main portion and defining respective triangular flaps to be folded onto the main portion.
A longitudinal sealing strip, formed when sealing the packaging material to form the vertical tube, extends along the pillow packs. Moreover, the top end portion and bottom end portion of each pillow pack have respective transverse sealing strips perpendicular to the longitudinal sealing strip and defining respective end flaps projecting from the top and bottom of the pack.
The top end portion and the bottom end portion of each pillow pack taper towards the main portion from the respective end flaps, and are pressed towards each other by a folding unit of the packaging machine to form flat opposite end walls of the pack, while at the same time folding the end flaps onto respective walls of the main portion.
Folding units are known, for example from EP-1726526, substantially comprising a chain conveyor for feeding packs continuously along a predominantly straight horizontal forming path from a supply station to an output station, and a plurality of folding devices which cooperate cyclically with each pack along the forming path to flatten the respective top end portion and bottom and portion of the pack and so fold the respective end flaps onto the top end portion and bottom end portion.
The folding unit comprises heating means arranged for heating the packs and melting the plastic material forming the outer plastic layer of the packs at the top end portion and bottom end portion.
In operation, there might be the need to pass from the production of a first type of packages to the production of a second type of packages, the packages of the second type of packages having substantially the same volume as the packages of the first type of packages, but a shape different from the shape of the packages of the first type of packages. In one case, the packages of the first type of packages may have a parallelepiped shape and the packages of the second type of packages may have non-rectangular side walls, for example side walls having curved, or polygonal, panels defining so-called design elements.
In order to switch from the production of the first type of packages to the production of the second type of packages, the folding unit has to be replaced with another forming unit.
The replacement of the folding unit with another folding unit requires a lot of time and a long stop of the packaging machine, which has a significantly high impact on the productivity of the packaging machine.